Floating spacecraft simulators (also known as planar air bearing test beds) are extensively utilized to conduct spacecraft dynamic model and guidance and control systems development, validation, and verification in a dynamically representative environment. In a floating spacecraft simulator, one or more robotic test vehicles, each of them representing a spacecraft, operate on top of a smooth, flat, and horizontally leveled surface. Planar air bearings are used to create a low friction loadbearing interface between the test vehicle and the flat surface. Given the smoothness and horizontally of the flat surface, the test vehicles experience a quasi-frictionless and weightless motion in two dimensions. This dynamic environment recreates the drag-free and weightless motion experienced by orbiting spacecraft (commonly referred to as reduced gravity or microgravity) on a plane.
The planar air bearings, which can be mounted either on the test vehicles or on the flat surface, use pressurized air to establish a thin film of air between themselves and their opposing surface. The air film acts as a lubricant and supports the weight of the test vehicle. The reduced gravity level attained on the test bed depends on the air bearings performance as well as on the flatness, smoothness, and horizontality of the operating surface. Epoxy-coated floors, glass panes, and granite tables are commonly used as operating surfaces. Granite tables are usually the preferred option as they can be accurately leveled and machined to a high planarity and smoothness level, while offering excellent stiffness and thermal stability.
Surveys on the technology and applications of air-bearing test beds are available. See e.g., Schwartz et al., “Historical review of air-bearing spacecraft simulators,” Journal of Guidance, Control, and Dynamics 26 (4) (2003); see also Rybus et al., “Planar air-bearing microgravity simulators: Review of applications, existing solutions and design parameters,” Acta Astronautica 120 (2016). See also, Wilde et al., “Experimental Characterization of Inverse Dynamics Guidance and Control in Docking with a Rotating Target,” Journal of Guidance, Control, and Dynamics 39(6) (2016); see also Ciarcia et al., “Near-optimal guidance for cooperative docking maneuvers,” Acta Astronautica 102 (2014); see also Curti et al., “Lyapunov-based Thrusters' Selection for Spacecraft Rotational and Translational Control: Analysis: Simulations and Experiments,” Journal of Guidance, Control, and Dynamics 33(4) (2010); see also Romano et al., “Laboratory Experimentation of Autonomous Spacecraft Approach and Docking to a Collaborative Target,” Journal of Spacecraft and Rockets 44(1) (2007). In addition to air-bearing facilities, neutral buoyancy, free-falling (either with parabolic flights or drop towers), and suspension apparatus for gravity compensation are also used in an attempt to recreate a reduced gravity environment. See e.g. Menon et al., “Issues and solutions for testing free-flying robots,” Acta Astronautica 60 (12) (2007); and see Xu et al., “Survey of modeling, planning, and ground verification of space robotic systems,” Acta Astronautica 68 (11-12) (2011).
Having ground-based facilities where the dynamics of orbiting spacecraft can be recreated is exceptionally valuable to develop, test, and validate spacecraft dynamic model and guidance and control systems. In particular, these test-beds are mainly used to recreate spacecraft docking and short duration close proximity maneuvers. Other potential applications, that would greatly expand the applicability of the test bed, include, for example, recreating spacecraft rendezvous, longer duration proximity maneuvers, and planetary landings. In these other applications, the dynamic environment to be recreated includes non-inertial, gravitational or other environmental accelerations that cannot be neglected. In principle, the test vehicle's actuators could be used to recreate these accelerations, but using the test vehicle's actuators interferes with the dynamic behavior of the test vehicle, reducing the dynamic fidelity of the test bed. See e.g. Ciarcia et al., “Emulating Scaled Clohessy-Wiltshire Dynamics on an Air-bearing Spacecraft Simulation Testbed,” AIAA SciTech Forum, Guidance, Navigation, and Control Conference, 9-13 Jan. 2017, Grapevine, Tex.
By tilting the test bed operating surface away from its nominal horizontally leveled state a gravitational acceleration is imparted to the test vehicles. This gravitational acceleration, if carefully controlled, can be used to recreate non-inertial, gravitational or other environmental accelerations. For example, the recreation of the surface gravity of a planetary body (e.g., asteroid) may be obtained to experimentally evaluate spacecraft landing maneuvers. By statically tilting the operating surface, a constant acceleration in a particular direction will be imparted to the test vehicle, thus recreating the planetary body's surface gravity. When recreating the relative motion of the two orbiting spacecraft in close proximity the resulting non-inertial acceleration is time varying and depends on the relative state between the two vehicles, thus requiring a time-varying operating surface tilt. Other environmental accelerations (e.g., solar radiation pressure, aerodynamic drag, gravitational acceleration while orbiting an irregular central body) are also be time-varying and require a time-varying operating surface tilt.
In current air bearing test bed setups, the orientation of the operating surface with respect to the local horizontal may, in some instances, be manually adjustable. Tilting the operating surface is then a manual, discrete operation, and the acceleration provided by gravity acts with constant direction and magnitude regardless of changing spatial and kinematic conditions among the test vehicle or vehicles present.
It would be advantageous to provide a floating spacecraft simulator comprising a flat table that dynamically and automatically changes its orientation with respect to the local gravity to impart a desired time-varying acceleration to the test vehicles. This capability could be used to impart time-varying accelerations to the test vehicles without using their actuators, thus allowing to recreate, for example, rendezvous, proximity maneuvers, and planetary landings, greatly extending the applicability of planar air bearing test beds.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.